The conventional method for introducing an electric field and varying the carrier density in a semiconductor channel is to use an electrostatic gate consisting of a gate dielectric layer and a metal contact layer over the semiconductor transport channel. Such gates are fabricated using standard lithographic techniques, and they are normally used to apply an electric field, but their geometry is fixed once they are fabricated. The intent is to control charge flow, not determine the luminescent or chemical sensing properties. Strain is normally introduced by mechanical means.
Ferroelectric films have recently been used as the gate dielectric in transistor structures because their polarization is non-volatile (it remains after the gate voltage has been removed). A ferroelectric transistor utilizes the nonvolatile, switchable polarization field of a ferroelectric gate to control the charge carrier density in the conducting channel. This approach is actively researched as an avenue to nonvolatile transistor memory, known as ferroelectric random access memory (FeRAM).
However, the conventional methods suffer from several drawbacks. The lateral size of such a gate is determined by the limits of the lithography used to produce it, and it cannot be changed once it is fabricated. In contrast, the ferroelectric polarization is non-volatile, and the position and lateral size of the ferroelectric domains can be changed after they have been fabricated.